Delayed Coking is a refinery process extremely important to convert petroleum residue or often called “bottom of the barrel” (usually made up of bottoms from atmospheric and vacuum distillation of crude oil) to important transportation fuels like gasoline and diesel. If unconverted, residue would be difficult to dispose off as fuel oil as it is too heavy. Delayed coking is important to improve economics of a refinery as it converts low value residue to high value transportation fuels. Delayed Coking involves thermal cracking where petroleum residue is converted to lighter products leaving behind a solid carbon product called “petroleum coke”. Thermal cracking occurs in a fired heater with horizontal tubes where residue is heated to predetermined temperature of about 925 to 950° F. Heated residue from the heater is sent to “Coke Drum” where it resides for hours and slowly converts residue to lighter products leaving “petroleum coke” in the drum. As complete coking takes place in the “Coke Drum” the process is termed Delayed Coking process. Three types of petroleum coke: sponge, shot and needle coke can be produced in delayed coking depending on the feed properties and the heater outlet temperature. Petroleum coke can be burned as fuel or converted to higher quality coke by calcining (heating to very high temperatures). Calcined coke is used in aluminum, chemical or steel industries. Calcined coke can be gasified to produce feedstock for petrochemical industry.
The feed heater is the heart of the Delayed Coking process. All heat is supplied in the heater tubes. The heater outlet temperature is typically about 925 to 950° F. at outlet pressure of about 40 to 60 psig.
As the technology evolved over years, latest heaters have two or four passes. Tubes are mounted horizontally on the side. Tubes are very long ranging from 2000′ to 4000′ depending on the design and are usually made of 9% chrome alloy. Burners are located on the floor of the heater and are fired upward. The burners of each heater box are controlled by the temperature of tube three or four back from the last tube to avoid fouling of thermocouples by coke. The heater is designed to have radiant section heat the feed and convection section to recover heat either with steam generation or other oil heating. Elliot (1996) states that the heater tubes be designed for an average radiant heat flux rate below 9000 Btu/hr-ft2 with mass velocity of 400 lb/sec-ft2.
As coke is formed on the heater tubes, heat transfer through coke to process fluid (in this case residue) decreases and heater outlet temperature begins to decrease below design temperature (between 925 to 950° F.). To maintain constant heater outlet temperature firing has to be increased, which raises the outer tube metal wall temperature. Ultimately coke thickness is so high that the outer tube metal wall temperature will reach its mechanical limit and then heater needs to be decoked. For 9% chrome alloy tubes, maximum tube metal temperature (max TMT) allowed is 1250° F. At the start of heater operation the tube metal temperature could be of the order of 1000° F. The time passed from start of heater operation (also called SOR) to end of heater operation (also called EOR) is termed heater run length. Heater run length could of the order of months depending on operating temperature and feed properties.
In the Delayed Coking process decoking is done by three methods: 1) Steam Spalling 2) Steam-Air decoking and 3) Hydraulic pigging.
Steam Spalling is done on-line by replacing residue in one of the passes with steam (in the form of boiler feed water). In on-line steam spalling, steam is heated to high temperature (about 1200° F.) and tubes are held there for predetermined period and then cooling the tubes by reduced firing to snap (or spall) the coke off the inside metal walls of the tube. Spalling occurs because tube metal walls contract more than coke resulting in stresses on coke which fracture the coke into pieces (called spalling). The steam and spalled coke go into coke drum. Steam spalling is recommended for 4 or more pass heaters. Steam spalling is not suitable for 2 pass heaters as large amount steam will go into coke drum causing process problems in the downstream fracionator.
Steam-Air Decoking: In this method, the heater is taken off-line and then steam-spalled first as described above. Steam spalling will remove majority of coke. Then small amount of coke left on the inside of tube walls is burned off by steam-air decoking.
Pigging: In this method, the heater is taken off-line and then steam-spalled first as described above. Steam spalling will remove majority of coke. The small amount of coke left on the inside of tube walls is scraped with Styrofoam pigs pushed by water pressure. Styrofoam pig is equipped with studs and grit on the external surface.
Today with most of the heaters being 4-pass designs, on-line spalling of one pass is done while other 3 passes continue to produce. The run length of any pass will decrease with each steam-spalling. For example at start of run (clean pass) run length may be 2 months but each successive run length will be lower and lower until last run length may be in days when pigging will need to be done to clean the pass (usually the whole heater) completely. Refineries report pigging being required in two to three years.
An industrial delayed coker feed heater is presented in FIG. 1. The delayed coker feed heater is divided into two sections namely radiant and convection sections. Radiant section contains a bank of tubes termed as radiant coils, heated by firing fuel on the outside to achieve desired coil outlet temperature for a given residue feed. The technology has evolved such that a small amount of steam (called velocity steam) is injected as boiler feed water in the radiant coil to increase the velocity of hydrocarbon through radiant tubes to prevent excessive coking Residue feed is heated in the convection section and enters radiant section at an incipient cracking temperature and exits at the predetermined coil outlet temperature usually between 925 to 950° F. Flue gases from the radiant section exit to convection section where the heat contained in the flue gases is primarily used to heat residue feed to incipient reaction temperature and balance is used to generate utility steam. Obviously incipient reaction temperature and coil outlet temperature varies with residue type and desired yield of products. During production, coke is formed on the inside of radiant tube walls. The thickness of coke formed is higher at the coil outlet than it is at the radiant coil inlet from convection section.
In order to prepare one pass of the delayed coker feed heater for decoking with on-line spalling method, valve F1 is closed and valve W1 is opened so that residue feed is cut off and boiler feed water is introduced. Boiler feed water converted to steam and heated to coil outlet temperature of about 1200° F. goes to the coke drum which is under production by receiving residue from other passes. After maintaining the pass at around 1200° F. (from 12 to 30 hours) firing is suddenly reduced to cool radiant tubes to temperature 800 to 1000° F. to induce spalling of coke from radiant tube walls. Steam flow is maintained so that spalled coke goes into coke drum.
In the on-line steam spalling, coil outlet temperature is maintained at around 1200° F. and steam entering radiant tubes from the convection section is at much lower temperature around 600° F. At these low temperatures steam cannot burn or gasify coke. However by injecting proper mixture of chemicals coke can be gasified at temperatures as low as 650° F. Obviously coke gasification rate increases with operating temperature.
Refining industry would greatly benefit if on-line chemical-steam decoking is used in the delayed coker feed heaters. In chemical-steam decoking water gas reaction takes place to gasify coke. Water gas reaction is endothermic which does not cause hot spots in the radiant coil. Accelerating water gas reaction to gasify coal or coke is probably the most investigated subject matter. Application of chemicals to gasify coke in ethylene radiant tubes (thermal cracking of hydrocarbons) has also been tried. For example, Kohfeldt and Herbert in U.S. Pat. No. 2,893,941 in year 1959 proposed injecting aqueous solution of K2CO3 into gas oil and steam mixture being cracked in ethylene furnaces. Coke produced by gas oil cracking (to produce ethylene) in radiant coils, will gasify by K2CO3. Kohfeldt and Herbert reported success in extending ethylene furnace run length. Kohfeldt and Herbert also observed that K2CO3 reduced coking in convection section banks which were operating at temperatures less than 700° F. Recently Gandman in U.S. Pat. No. 6,228,253 proposed injecting aqueous mixture of potassium carbonate and magnesium acetate in ethylene furnace radiant coil or coils where steam decoking effluent along with cracked effluents from other coils were routed to recovery area similar to Kohfeldt and Herbert. Gandman reported success in removing coke from the selected radiant coils. Stancato and DeHaan in 2001 reported at the Ethylene Producers Conference, that rate of gasification of coke by K2CO3 is 16 times the rate of gasification by steam alone.
Main products of water gas reaction in decoking are CO and H2 with a small quantity of CO2. In the delayed coker process CO, H2 and CO2 are produced in normal production. Thus when one pass of the heater is under decoking the products of decoking are not harmful to the primary products in other passes going to the Coke Drum.
Success of applying chemical-steam gasification to Delayed Coking feed heater depend on proper marriage of steam decoking process parameters such as steam flow rate and coil outlet temperature with chemical injection rate.
Patents and literature are full of many investigations of chemicals used in accelerating water-gas reaction. We researched literature to find another reasonably priced chemical which can provide synergy with K2CO3. In a comprehensive review of carbon gasification, Nand (1981) in his thesis refers work of Kayembe and Pulsifier (1976) who found that the highest steam gasification rates were achieved by K2CO3 and KOH.